Permanent magnet properties of bulk magnetic materials having large magnetocrystalline anisotropies can be enhanced by reducing them to powders. Such powders can be incorporated in bonding media to provide composite permanent magnets having properties substantially superior to those of the bulk source materials. Powders can be prepared by grinding or by chemical means. It is common practice to add plastic to magnetic particles by adding polymer solution to the powder and mixing. The solvent is later removed leaving large pieces of briquettes of randomly oriented material. This material must be reground to a powder before being subjected to a pressing and alignment cycle. However, powders have a large surface area per unit volume and, therefore, tend to be reactive. For example, if a powder of cobalt-rare earth material is exposed to air its coercive force will decrease irreversibly due to the oxidation of the particle.
Since the reactivity of the powder particles appears to be a surface phenomenon, efforts have been directed toward reducing the reactivity by coating the surface with a protective material. One way to accomplish this is by applying a coating of zinc or arsenic as disclosed and claimed in Becker et al. U.S. Pat. No. 3,615,914, which is assigned to the same assignee as the present invention.
Once the cobalt-rare earth particle is protected by a metallic coating such as zinc, it is mandatory that this coating be unaffected by abrasion, or cleavage of the particle. Therefore, the common technique of regrinding the bulk magnet-plastic binder composite is not desirable for highly reactive materials such as cobalt-rare earth particles because of the abrasion and cleavage of particles which takes place during this operation.
The present invention has for its object to provide a method for protecting the surface of magnetic powder material from changes which would degrade the magnetic properties of the material. Another object is to provide a method for coating a magnetic particle which does not need to be subjected subsequently to grinding. A further object is to provide magnetic powder particles with a surface which will serve as a lubricant to help achieve maximum packing density without serious abrasion during a subsequent hot-pressing step. An additional object is to provide magnetic particles with a polymer coating which will serve to hold the aligned magnetic particles together after pressing.